1. Field of the Invention
The invention disclosed herein relates to measuring porosity of an earth formation and, in particular, to measuring with neutron porosity tools.
2. Description of the Related Art
Exploration and production of hydrocarbons requires that accurate and precise measurements of earth formations, which may contain reservoirs of the hydrocarbons. A borehole is typically drilled into an earth formation for the exploration and production.
Well logging is a technique used to perform measurements of an earth formation penetrated by the borehole. In one embodiment, referred to as logging-while-drilling, a logging tool is used to perform the measurements during drilling or during a temporary halt in drilling. The logging tool is attached to a drill string having a rotary cutting device. Thus, as the drill string rotates to turn the rotary cutting device, the logging tool also rotates with the drill string.
Various types of measurements can be performed using a logging tool. One type of measurement is used to estimate a porosity of the earth formation penetrated by the borehole. This type of measurement in one embodiment irradiates the earth formation with neutrons and measures an amount of neutrons reflected by the earth formation. Because an amount of reflective material present in the pores of the earth formation can determine the amount of neutrons that are reflected by the earth formation, the amount of reflected neutrons provides an estimate of the porosity of the earth formation. Thus, it is important to accurately measure the amount of reflected neutrons in order to accurately estimate the porosity of the earth formation under investigation.
Several challenges may occur in attempting to detect and/or measure neutrons in a borehole environment. One challenge is presented by a borehole fluid or drilling mud. The borehole fluid is present external to the logging tool. The drilling mud flows through a mud channel internal to a drill string to which the logging tool is attached. The borehole fluid or drilling mud can slow down neutrons for detection independent of the porosity of the formation. Thus, the amount of neutrons measured may not represent a correlation to the porosity of the formation.
To overcome the effects of the borehole fluid or the drilling mud, conventional logging tools may wrap a thin foil that absorbs neutrons around a neutron detector followed by a reflector material wrapped around the absorber foil. By optimizing the thickness of the absorber foil, the thermal neutron contribution to the detector from the borehole fluid or drilling mud would be mildly reduced and, at the same time, improve its detection efficiency to the neutrons from the formation. However, further reduction of the neutron's contribution from the borehole fluid or the drilling mud is nearly impossible with the conventional design. The reduction is nearly impossible with the conventional design because in order to reduce the contribution from the borehole fluid or the drilling mud, a relatively thicker absorber would be necessary. On the other hand, increasing the thickness of the absorber foil would result in fewer neutrons being reflected back because neutrons reflected back from the reflector material will have to penetrate the absorber foil twice without being absorbed.
Therefore, what are needed are techniques for significantly reducing or eliminating detected neutrons reflected from a borehole and/or mud channel and, therefore, accurately measuring those neutrons reflected only from an earth formation. Preferably, the techniques can be used in a logging tool disposed in a borehole penetrating the earth formation.